Enables spermine synthase activity. Involved in spermine biosynthetic process. Is expressed in several structures, including anterior endoderm; dorsal pharyngeal muscle cell; embryonic/larval midgut; embryonic/larval midgut primordium; and head mesoderm. Used to study syndromic X-linked intellectual disability Snyder type. Human ortholog(s) of this gene implicated in syndromic X-linked intellectual disability Snyder type. Orthologous to human SMS (spermine synthase).